Found in a 1966 documentation, this “coin finder” uses components from the era operating at 2.1 MHz. The circuit uses an audio amplifier for detection.
This is the circuit published in the August 1973 issue of Radio Electronics magazine announcing its...
This circuit can be used to add a delay to a logic signal as shown by the input and output...
This tone control was obtained in the Audio Handbook of the National Semiconductor of 1977. The...